All electrical machines generate heat during their operation. This heat is detrimental to the operation as overheating decreases the operational efficiency of a machine and may even cause damage. In extreme cases, overheating of an electrical machine can cause complete failure. It is therefore necessary that all electric machines are cooled. At the simplest level, small low-power electrical motors may be cooled sufficiently through thermal contact with their surrounding environment. However, most electrical machines require a much greater degree of cooling and therefore incorporate a suitable cooling system. There are many different cooling systems that are currently in use. For example, machines may be cooled by a closed circuit carrying a liquid coolant to a heat exchanger or by direct gas cooling. The specific cooling system that is incorporated with any electrical machine will depend on the size, power and construction of the machine, among other factors.
Cooling systems inevitably add to the size, weight and cost of the electrical machine. Cooling can be a particular problem for electrical machines that are required to be particularly small, light and low cost, for example wind turbine generators. In such machines it is preferable to minimize the size and weight of the cooling system. Therefore it is often preferable that the cooling systems of such machines utilise the surrounding air to cool the machine. This is because such systems are not required to contain and circulate large quantities of heavy liquid coolant, which can add significantly to the weight and size of the cooling system. It is even more preferable that surrounding air is used to directly cool such machines without the use of bulky and heavy heat exchangers. Direct cooling of electrical machines by the surrounding air is often known as open-ventilation.
Open-ventilation is a very efficient cooling method for electrical machines since there is no intermediate heat exchange system which would otherwise reduce the available temperature rise for the machine. However, open-ventilation systems are not commonly used without some form of protection because the surrounding air used to cool open-ventilated electrical machines often carries dirt dust, moisture; salt and other impurities. If the surrounding air does contain substantial quantities of airborne particles then significant damage can be caused to the exposed parts of the machine during passage of that air through the open-ventilation system. The damage is caused by the particles colliding with the exposed parts of the machine. This problem is compounded by the fact that, in order to increase the degree of cooling provided by open-ventilation systems, it is preferable that the cooling air is forced by mechanical means to pass through the electrical machine at a relatively high-velocity and/or pressure. This can make the damage caused by particles colliding with exposed parts of the machine much worse. As a result, it is usually necessary that open-ventilation systems incorporate a means of removing at least some of the particles from the air that enters the electrical machine.
Currently, in order to prevent particles from entering open-ventilated high-power electrical machines, they are usually contained within box-type enclosures. These enclosures allow open-ventilation of the electrical machine via blow-through inlet and outlet chambers. The most common are National Electrical Manufacturers Association (NEMA) II-type enclosures. In these enclosures the incoming air is channeled in ducting through at least three 90° changes of direction and into a low velocity air chamber before entering the machine. This has the result of separating out a proportion of the airborne particles present in the incoming air before the air enters the electrical machine itself.
Although box-type enclosures do remove a proportion of the particles from the incoming air, they have been found not to be effective enough for many situations. For example, machines with NEMA II-type enclosures have particularly suffered in desert situations, where the surrounding air may contain particularly high quantities of solid particles, especially sand. Furthermore, mechanically increasing the velocity of the air passing through a box-type enclosure reduces its efficiency in removing particles from the air. Box-type enclosures also have a problem with the build up of particulate matter which has been removed from the air passing through the enclosure. This is because, as the enclosures clean the air in a passive manner, relying on changes of direction and a low velocity air chamber and have no active way of ejecting the particles back into the surrounding air, a relatively large proportion of the particles that are removed from the air simply build up within the enclosure. Over time this leads to a decrease in the efficiency of operation of any enclosure. It also means periodic cleaning of box-type enclosures in order to remove particle build-up is necessary.
Filters are also commonly used to clean incoming air in open-ventilated machines. Filters are used either in isolation, as the sole method of cleaning the air, or integrated with other air-cleaning devices. For example, filters are commonly incorporated in the box-type enclosures discussed above. Typically, filters are comprised of simple mesh screens that are located in positions such that all the air entering the open-ventilation system must pass through at least one filter. Large particles present in the incoming air cannot pass through the filter or filters and are thereby prevented from passing throughout the electrical machine.
Some electrical machines have more than one set of filters. Specifically, some machines may have open-ventilation systems formed such that the incoming air first passes through a relatively coarse filter designed to remove larger particles and then through a progression of finer filters, each designed to remove smaller particles than the immediately preceding filter.
Upon initial use, filters are reasonably effective at removing large particles from air entering an open-ventilation system. However, their effectiveness deteriorates with time, especially if they are not regularly maintained. The rate of deterioration can be particularly hard to predict as it depends upon external factors, including the quantity and nature of any particles present in the incoming air. Furthermore, some filters such as box-type enclosures, only passively remove particles from the incoming air. That is, a relatively large proportion of particles removed from the incoming air by the filters simply accumulate within the open-ventilation system. It is also very difficult to use filters to remove extremely small particles from incoming air, particularly if a high air velocity and/or pressure is utilised within an open-ventilation system.
Due to the disadvantages described above, neither box-type enclosures nor filters, or even a combination of the two, provides a reliable method of removing airborne particles in environments where the surrounding air has high particle content or where the air passes through an open-ventilation system at high-velocity and/or pressure. There is therefore a need for an improved air cleaner apparatus for removing particles from incoming air in open-ventilated electrical machines. Such an apparatus is required to be small and light-weight, and preferably low in cost. It is preferable that any such apparatus is capable of being incorporated with current conventional enclosures and open-ventilation systems. It is also preferable that the apparatus is capable of use with high-velocity and/or high pressure open-ventilation systems and in environments where the air may contain large amounts of particles. It is also desirable that such an apparatus actively removes the particles from the open-ventilation system in order to prevent the build up of such particles within the system.
As a result of this need, it has previously been proposed to use centrifugal force, as applied by a fan or other such rotating means, to remove solid or liquid particles from air entering an open-ventilation system. One example of such an apparatus is disclosed in JP 56125950. In this apparatus air is allowed to enter an open-ventilation system from the surrounding environment, the solid particles present in the air are removed and the air is then channeled into an electric machine. Specifically, air is allowed to enter the open-ventilation system via an inlet formed in the centre part of the front side of a filter frame. The incoming air then enters a conical passage where it is subjected to a rotary force. Solid particles present in the incoming air are thrown radially outwards into a dust collecting chamber surrounding the conical passage. The cleaned air then passes through a filter and into the electric machine. In this manner, the open-ventilation system of JP 561 25950 provides active cleaning of incoming air. However, this system does not expel the particles that are removed from the incoming air. Instead those particles are collected within a dust collecting chamber. Therefore periodic emptying of the dust collection chamber is necessary.
An electric machine incorporating a similar open-ventilation system is disclosed in GB 1106589. In this system cooling air is drawn into a chamber that is formed at one end of the casing of the electric machine. The air entering the chamber is immediately rotated by a fan that is mounted on the drive shaft of the electric machine and that also acts to draw the surrounding air into the open-ventilation system. Solid particles present in the incoming air are thrown to the radially outer edge of the chamber as a result of the centrifugal force applied by the fan. These particles then enter an outlet air stream rather than passing through the machine. The outlet air stream is formed only at the radially outer edge of the chamber. Therefore, the majority of the incoming air is allowed to enter and circulate around the machine. In this manner the electrical machine of GB 1106589 provides active cleaning of incoming air. This machine also expels solid particles cleaned from the incoming air completely out of the machine. This means that the expelled particles cannot build up within the machine and decrease the efficiency of its operation or cause it damage.
The open-ventilation system of GB 1106589 has a number of problems. First, because the fan that is providing the centrifugal force to the incoming air also acts to draw the air into the machine the two processes are inseparable. This means it is not possible to independently regulate the centrifugal force applied to the incoming air and the volume of air that is drawn into the machine, as may be desired. Furthermore, although the particles that are removed from the incoming air are generally thrown towards an outlet air stream, there is little or no active force which draws those particles into the outlet air stream, other than the centrifugal force from the drive-shaft fan. As a result, it is likely that a relatively high proportion of the solid particles entering the open-ventilation system of GB 1106589 will not be removed from the system and will be circulated around the electric machine.
Due to the problems discussed above, open-ventilation systems that utilise centrifugal force to remove solid particles from incoming air have not been employed for use on electrical machines on a commercial scale. This is despite the concept being known for many years. For example, the basic concept was disclosed as long ago as 1964 in GB 977042.